Conventional transformers are employed widely for matching from the optimal impedance at a differential Power Amplifier (PA) output to a desired system impedance level. A typical block diagram of such an amplifier is illustrated in FIG. 1.
FIG. 1 illustrates a conventional power amplifier 10 employing a balanced to unbalanced transformer (a “balun” transformer) to perform a desired impedance match.
In the primary side of FIG. 1, amplifier 12 is a differential power amplifier, powering port (or positive node) 14 and port (or negative node) 16. These are balanced ports, or balanced nodes. Primary capacitor 18 links port 14 to port 16. Primary inductor (or coil or winding) 20 also links port 14 to port 16. Thus, primary capacitor 18 and primary inductor 20 are in parallel. Dot 30 marks the top side of primary inductor 20. Throughout this specification and claims, the adjective “top” refers to a dotted end of an inductor. Using a differential power amplifier 12 yields a balanced power output, such that a virtual ground exists in the center of primary inductor 20.
In the secondary side of FIG. 1, secondary inductor 22 is magnetically coupled with primary inductor 20, such that the dotted ends (top ends) of these inductors are in phase. The top of secondary inductor 22 is directly linked to port 24, which is an unbalanced port. A bottom of secondary inductor 22 is directly linked to ground 28, which causes the unbalanced port. Secondary capacitor 26 is linked in parallel to secondary inductor 22.
Both primary and secondary coils (primary inductor 20 and secondary inductor 22) are resonated with capacitors (primary capacitor 18 and secondary capacitor 26 respectively) to present real impedances at the frequency of interest. In a cellular PA (power amplifier) tuned for high efficiency, the harmonic levels at the unbalanced output port 24 must later be filtered (not shown) to meet the ETSI (European Telecommunications Standards Institute) standards at the antenna. These standards and the associated filter losses result in a direct trade-off between the overall efficiency of the amplifier and the harmonic levels.